This invention relates to an apparatus for cutting spirally formed hollow metal pipes. More particularly, the present invention relates to a pipe cutting apparatus having non-rotatable knives that overlap to cut a rotating pipe.
Hollow metal pipes are widely used for ventilation ducts. These pipes are formed from a continuous strip of thin metal. U.S. Pat. No. 4,567,742, issued Feb. 4, 1986, describes a preferred machine for making triple-ribbed, spiral seam pipe. U.S. Pat. No. 3,132,616 (Hale) describes another type of machine for making corrugated, spiral seamed pipe. Both types of machines start with a flat strip of metal. The strip passes through a series of rollers which bend the edges into predetermined shapes, and form parallel corrugations or reinforcing ribs in the strip. The strip then passes around the inner surface of a forming head (a mandrel) in a spiral manner, so that the strip takes a spiral shape with opposite edges of the strip meshing. The intermeshed edges of the strip are then compressed to form a pipe with a spiral lockseam. The strip is continuously fed into the machine to continuously produce spiral seamed pipe.
The pipe must be cut when it reaches a desired length. Previously, some pipe cutting machines used a rotary saw for cutting the pipe. In one machine, the saw is mounted outside the pipe, where it does not interfere with the pipe forming process. When the pipe is ready to be cut, it stops moving. The saw blade is then moved into its cutting position and penetrates the pipe. The metal strip material and pipe forming operation then begin again at a slow speed. This causes the pipe to move forward and rotate. The saw is adapted to move axially with the pipe for one complete rotation, whereupon the pipe is completely severed. The saw returns to its starting position, and the cut pipe section is discharged onto a run-off table. The tube forming process is then repeated to produce another section of pipe.
There are several disadvantages to using high speed saws to cut metal pipes. First, the saw blade, which rotates as fast as 5000 RPM, is dangerous to the machine operator and to anyone near the machine. Second, the saw generates a lot of sparks when it cuts the metal, which also creates a hazardous situation. Anyone near the machine must wear safety glasses to protect himself from the sparks. Third, the saw cut leaves burrs on the edge of the pipe. These burrs must be filed off by someone, who must be careful not to cut himself on the burrs. Finally, the cutting operation is very noisy.
Another apparatus for cutting spirally formed pipe uses passively rotatable knives. One rotatable knife is positioned inside the pipe and a second rotatable knife is positioned outside of the pipe. When cutting, the outer knife moves into an overlapping position with the inner knife so that the knives rotate as the pipe rotates and is cut between them. The rotatable knives each require a bearing assembly. Typically, bearing assemblies require multiple additional parts configured in a precise manner.
Accordingly, a pipe cutting apparatus is desirable that overcomes the disadvantages of a high speed saw and is less complex and less expensive than cutters having rotatable knives with individual bearing assemblies.